Method and system of circuits for overcoming static effects



Aug. 7, 1934. CRAWFORD 1,968,817

METHOD AND SYSTEM OF CIRCUITS FOR OVERCOMING STATIC EFFECTS Filed Aug. 12, 1930 //v vdv TOE ATTORNEY Patented Aug. 7, I934 general l wave :form as ilther disturbing pulse or" of l METHOD "SYSTEM OF "CIRCUITS FOR 1 OVEBCOIYHNGSTATIG EFFECTS if beY r i i i w i -M fll tq m A 'pplication dugust 12,1930, Serial No. 474,791

"3 Claims. (01. 250 -2 a iii-March 3," 1883, as

This inventiondescribed hereinmay bemanufactured and ,used by or for the Government forgoyernmentalpurposes, without the payment to, me of any royalty thereon, p n a j This invention relates to receiving circuitsnin telephonic and, telegraphic apparatus and itsjpri-g mary object is to provide a method and system: of circuits bywhichit is possible-to differentiate the desiredsignal tram-Ethe effects of atmospherics" or other disturbing electrical forces acting upon thesystem. r-

OI1B of the objects of this inventionis to provide. a method of the characterdesignated"hereinafterby which:theantennacircuit of a radio receivingstationor set is electrically conditioned so that it. cannot be materially knocked into osoillationiat the desired signal frequency,=-whereby'anydis-w' turbin-gimpulse striking the antenna will set up currents in the main antenna circuit of:the same frequencies other-thanthatof the desired signal; either of which may be readily separated from the. desired signal. t, 7 '4 ,Anotherobject of, this invention isto effecta 2 methodand system ofcircuitsby which the voltageimpressed upon the gridof thefirst tube of a: receiving device ,Will "be .materially greater at, signal frequency, than at. the .frequencyi of the effects of the distributing:forcei i a 1 Another object of this'invjention is to.:providef a method and system of circuitsgwherein the-plate current variations at the desiredsignal frequency are transmitted without; material 2 reduction to succeeding amplifying circuits; vwhile the platen current variationsat the frequencyiof the disv turbing electrical effects are..materiallya-reduced thereinw g r: w w?" a Another object of this inventionis to produce' a method and system of 'circui'tswhereinnotonly the mainantehfiaLtirChitis'protectd against being knocked into oscillation by the disturbing force" but-likewise all tunied circuits associated with n An h r i ti x li ent nis to pro idea J hodandsystem e'ircuit's wherein aparalleL resonance circuit including inductance and lea-I; V pacity i ip l n ert d n Series in the 'nia in anten, ajcircuit so that. when the parallel reso w nan e" circuit is turned to the desired .signalffrecurrent" l flowinvt'he main l a careful studyof. thehnatiire of the disturbance,

would be necessary to definitely determine its characterizing features, andjwith this in view numerousobservations were taken over a number of years: in localities where static conditions were bad-, such" as for-instance in the Panama Canal ZonezzIThe 'results of these tests indicated that static disturbances were in great part of the nature of impulses'of extensive amplitude but of comparatively short duration, in. other words, waves either. unidirectional or of relatively low frequency.

With these data as a basis further experimentationdemonstrated that interference which the effects or these disturbances developed in reception was largely attributable to impact oscillations setup in thei'antenna circuit or in the tuned circuits connected withit: The establishment of thesefacts led to the conclusion that if the main antenna circuit aswell as the tuned circuits constituting the receiving system could be protected against being knockedinto, oscillation at signal frequency,- 'one. O-fi the major diificulties of this troublesome problem of static elimination would bemetgbecause if the main antenna circuit could not belmateriallyxkno'cked into oscillation at the desired signalfrequency the impinging staticimpulse would set up currentsin the main antenna circuit correspondingingeneral wave form to that "of the disturbances or at'frequencies other than the desired signalfrequency, either of which might be disassociated from the desired signal.

work-on this problem developed it became evident I that by including suitable attenuating meansnincthe antenna circuit and providing bypassingfacilities i'n-the amplifier plate circuits thecurre'nts at other-than desired signal frequenciesandthe amplifications at such frequencies could readily be eliminated ormaterially reduced, andthatfurther if the-tuned circuits of the system as well as battery, filament leads and etc., were properly shielded these circuitsnwould be protected from being knocked into oscillation or tenna circuit sothat it could not be knocked into osc i ationatsi gnalzirequency; recourse was had to arallel resonance circuit consisting of a qwz esetane i ductan e e d a. ow res stance capacity in parallel, usually referred to as a trap. When a circuit of this character is inserted in series in the main antenna circuit and tuned to the desired signal frequency there will be little or no current flow in the main antenna circuit at the desired signal frequency owing to the fact that the currents flowing through the inductance and capacity branches of the trap are of equal strength and having a phase difference of 180 they tend to neutralize each other as far as the main antenna circuit is concerned, thus precluding material current flow at signal frequency except in the trap.

There being little or no flow of current in the main antenna circuit at signal frequency, then, when such a circuit is impinged upon by a static impulse, currents will be set up in the main antenna circuit corresponding in general wave form to that of the disturbing voltage impressed on the antenna or of frequencies other than the desired signal frequency. From this it follows that a trap circuit inserted in series in the antenna circuit and tuned, not to the frequency of the disturbance, but to the desired signal frequency will function to protect the antenna from being materially knocked into oscillation at the desired signal frequency.

Having brought about a condition of the main antenna circuit whereby at the desired signal frequency, there is little or no current flow in said circuit other than the low or other than desired signal frequency currents produced by the disturbance, the next important feature to be considered is the elimination or reduction of these currents and the effects thereof on the plate circuits of the several stages of radio frequency amplification associated with the main antenna circuit. To do this suitable resistance, inductance or capacity or combinations thereof may be inserted in the main antenna circuit, while the plate circuit of the first tube should be provided with means which will cause the amplified currents at other than the desired signal frequency to be by-passed, but permit the variations of plate current at desired signal frequency to pass to the primary of the radio frequency transformer. It may also be desirable to include similar plate circuit arrangements in succeeding tube circuits.

The foregoing requirements having been complied with there remains but one more important factor to be considered, namely, protection of the several tuned circuits connected with the main antenna circuit against being knocked into oscillation and this may be accomplished by thoroughly shielding these circuits as well as all battery leads, filament leads, etc., with any suitable material ordinarily employed for this purpose.

As the preceding discussion merely outlined the fundamental principles underlying this invention, recourse will now be had to a specific embodiment thereof shown in Fig. 1 of the drawing which illustrates a. system of circuits, comprising a main antenna circuit, a trap circuit in series therewith, two stages of radio frequency amplification and a fragment of a succeeding stage, all of which if desirable, may be employed in a complete system of reception.

Reference being had to the illustrated embodiment of this invention, A indicates a main an-- tenna circuit in which there is inserted in series fixed or variable resistance (1), variable or fixed capacities (2) and (3) of low value of the order .0001 to .00025 mid. and a high resistance (10) shunted around the capacities (2) and (3) for a purpose hereinafter to appear. B represents a trap circuit which is inserted in series in the main antenna circuit and includes fixed or variable capacity (4) and inductance (5).

One or more stages of radio frequency amplification may be associated with the antenna circuit, but in the present instance two complete stages only are shown which include amplifying tubes C and D each including a. grid (6), plate (7). and a filament (8). The grid (6) of the input tube is shown as being directly connected to the trap by the lead (9) although it is to be .understood the connection may be inductive or capacitive if desired. Filament (8) is energized by filament current supplied by a source of E. M. F. (11) while plate (7) is connected to the plate circuit (12) and supplied by a source of E. M. F. (13).

The plate current variations at signal fre quency are transferred from the first stage of radio frequency to the next stage thereof by means of a transformer E, the primary (14) of which is connected to the plate circuit of the first tube and the secondary (15) to the grid (6) of the tube D. As the plate circuits of tube D are exactly identical to those of the first tube they will hereinafter be identified by corresponding reference characters.

In order to by-pass the plate current variations of relatively low disturbance frequencies the plate circuit of the first stage of amplification should include a condenser of small capacity (16) which may be variable or fixed in value, and a radio frequency choke (17) connected across the circuit in advance of the condenser as clearly shown in Fig. 1 of the drawing. Similar plate circuit arrangements may be used for succeeding tubes if their corresponding grid circuits are untuned or very broadly tuned. In Fig. 1 such an untuned grid circuit is shown for tube D.

The preceding system of circuits defines one form of this invention by which it may be rendered effective, but it is to be understood that various other circuit arrangements and various combinations of circuit values may be resorted to without departing from the fundamental prin ciples hereinabove laid down. For example, capacity (4) may be relatively large and inductance (5) relatively small and damping resistance (18) may be added in either or both branches of the antenna trap to increase the natural decrement of the trap circuit.

With the main antenna circuit, the trap circuit and the several stages of untuned, broadly tuned or tuned radio frequency associated as described the application of this invention for overcoming static effects is as follows:

Variable condenser (4) is adjusted so as to tune the trap circuit B to the desired signal frequency whereupon (for the reasons previously given) little or no current flow will occur at signal frequency in the main antenna circuit and consequently when the latter is impinged by a static impulse, or other disturbing electrical forces, currents will be set up in the main antenna circuit having a wave form corresponding to that of the impulse or of frequencies other than the desired signal frequency.

When these currents reach the trap B they will divide between its capacity and inductance branches, but since the IXL drop across the trap at disturbance frequencies is relatively low compared to the 1X1. drop across the same trap at signal frequency, the ratio of voltage drop at signal frequency to that at disturbance frequency is materially increased.

Returning for a moment to the main antenna circuit and having in mind there is little or no current flow in said circuit at signal frequency, it will be apparent that the impedance of the resistance (1) and capacities (2) and (3) at signal frequency will be low, while for the currents set up by the static impulse, their impedance will be high, consequently this resistance and capacity will produce a material reduction of the disturbance voltage applied to the grid of the first tube.

Coming now to the radio frequency amplifiers, as the two classes of voltages are impressed upon the grid of the first tube, namely the disturbance voltages and the voltage at signal frequency} current variations will be set up in the plate circuit at corresponding disturbance and signal frequencies. In order to reduce the effective variations of the plate current at disturbance frequencies, a condenser (16) is arranged in the plate circuit so that said variations at disturbance frequencies, due to the relatively high reactance of the condenser, will be by-passed through the radio frequency choke (17) and thus produce little or no effect on the grid of the second tube. On the other hand the variations at signal frequency will readily pass through condenser (16) to the primary (14) of transformer E and be transmitted from the secondary (15) thereof to the input of the next stage. As this same action occurs in succeeding untuned or very broadly tuned stages it is thought the description with respect to the first stage will be sufficient for the purpose of this disclosure.

In the preceding description a variable condenser (4) was utilized to aid in pointing out the action of the trap B. Since the natural decrement of such a circuit is greater if the capacity of condenser (4) is large, it may be of advantage in particular instances to make condenser (4) of a suitably large value and the inductance (5) of correspondingly suitable small value and to insert a resistance (18) in either or both branches of the antenna trap to further reduce the static effects. It is not intended to limit the application of the foregoing principles to a continuously variable condenser (40.

Up to this point consideration has been given primarily to reducing the effects of low frequency impulses. If, however, a highly damped high frequency disturbance of other than signal frequency should impinge upon the antenna the resistance and capacity or both in the main antenna circuit will again materially reduce the voltage applied to the grid of the first tube as hereinbefore described in connection with low frequency impulses. For a frequency materially greater than that of the desired signal the majority of the current will flow through the capaclty branch of the trap and the IXc drop across the trap will be considerably lower than that of the desired signal.

In the event that static charges should be impressed upon the main antenna it has been found advisable to bridge the capacities (2) and (3) with the high resistances (10) in order to provide a high resistance leak to ground.

In conclusion it is evident that this invention provides a' method for overcoming or reducing the effects of static disturbances involving certain fundamentals, namely, prevention of the main antenna circuit from being materially knocked into oscillation at signal frequency, reduction of the disturbance voltage applied to the input of the first tube, elimination or reduction of the variations in the plate circuit at disturbance frequencies and protection by suitable shielding of all tuned circuits associated with the main antenna against being knocked into oscillation, and if these principles are applied to any receiving apparatus whether similar to that shown and described or otherwise, it will prove effective.

Having described my invention, what I claim as new and wish to secure by Letters Patent is:

l. A method for overcoming the effects of disturbing impulses on a system of reception, the latter including a main antenna circuit and a parallel resonance circuit in series therewith, said method consisting in tuning only the parallel resonance circuit to signal frequency so as to prevent the main antenna. circuit being materially knocked into oscillation at signal frequency, so that impulse currents set up in the main antenna circuit will have wave forms corresponding to those of the disturbing impulses and will be at frequencies other than that of the desired signal and effecting an attenuation of said impulse currents.

2. A method for overcoming the effects of disturbing impulses on a system of reception, the latter including a main antenna circuit and a parallel resonance circuit in series therewith, said method consisting in tuning only the parallel resonance circuit to signal frequency so as to prevent the main antenna circuit being materially knocked into oscillation at signal frequency, so that impulse currents set up in the main antenna circuit will have wave forms corresponding to those of the disturbing impulses and will be at frequencies other than that of the desired signal and then by-passing the currents at other than signal frequency to separate them from those at signal frequency for reception purposes.

DAVID M. CRAWFORD. 

